What is the main conflict in The Last Picture Show by Larry McMurtry?

In the mid-1950s, Sonny Crawford and his best friend Dwayne are high school seniors in the small town of Thalia, Texas. Both boys play football, hang out at the pool hall and obsess about sex. Dwayne dates Jacy Farrow, the richest, prettiest girl in town, and Sonny dates Charlene Duggs, an unattractive and unpleasant classmate. As the novel begins, Sonny is overwhelmed by the loneliness of the town and his dissatisfaction with his dating prospects. Sonny dumps Charlene after a night at the picture show and begins an affair with Ruth Popper, the football coach's attractive but neglected wife. Sonny enjoys having sex with Ruth and accepts that she is in love with him.